More than two-thirds of the US population was under an extreme weather alert on Friday amid an Arctic winter storm - with temperatures forecast to drop as low as -45C in some parts this week.
The deep freeze has seen thousands of flights cancelled across the US and Canada causing travel chaos in the run-up to Christmas, while homes and businesses have been left without power, and at least three people have died.
Governors of New York, Kentucky, North Carolina, West Virginia, Georgia and Oklahoma have all declared states of emergency.
With a column of bitter cold that stretched from Texas to Montana starting to march eastward, more than 240 million people were under weather advisories on Friday, the National Weather Service said.
Hard-freeze warnings were posted in parts of the Southern states of Louisiana, Alabama, Florida and Georgia.
As North Americans are braced for their coldest Christmas in decades, the National Weather Service has warned temperatures could be cold enough for frostbite to develop in five to ten minutes.
Frostbite can lead to body extremities not getting enough blood, which can lead to tissue rupturing with, in extreme cases, amputation being required.
The service has warned of a bomb cyclone striking on Friday - a term used for when a midlatitude cyclone intensifies at speeds of at least 24 millibars, the measure of atmospheric pressure, over a 24-hour period.
US president Joe Biden said in a briefing: “This is not like a snow day, when you were a kid, this is serious stuff.”
Even traditionally warmer states such as Florida and Texas are being subjected to extreme weather. El Paso in Texas, where migrants are sleeping on the streets, could suffer lows of -9.4C.
New York state governor Kathy Hochul warned of ice jams, which happen when large pieces block the flow of a river and can cause flooding. “[Ice jams] are going to wreck havoc in our community.”
Power outages were expected from high winds, heavy snow and ice, as well as the strain of higher-than-usual energy demands.
One of the greatest immediate impacts, even before the storm fully took shape, was the upending of commercial air traffic during the busy holiday travel period.
More than 5,000 US flights scheduled for Thursday and Friday were cancelled, with two major airports in Chicago accounting for nearly 1,300 of the cancellations, according to flight-tracking service FlightAware.
It said more than 3,400 flights within, into or out of the US were cancelled on Friday alone , according to the tracking site, causing more mayhem as travellers try to make it home for the holidays. Some airports, including Seattle-Tacoma International Airport, closed runways.
At least 1.4 million homes and businesses were without electricity on Friday morning.
The cold also led to a high demand at homeless shelters, including in Detroit, where some shelters were at capacity on Thursday.
And in Portland, Oregon, officials opened four emergency shelters.
Courtney Dodds, a spokeswoman for the Union Gospel Mission, said teams from her organisation had been going out to try to persuade people to seek shelter.
"It can be really easy for people to doze off and fall asleep and wind up losing their lives because of the cold weather," she said.
In famously snowy Buffalo, New York, forecasters predicted a "once-in-a-generation storm" because of heavy lake-effect snow, wind gusts as high as 65mph, whiteouts and the potential for extensive power outages.
Mayor Byron Brown urged people to stay at home, while the NHL hockey league postponed the Buffalo Sabres' home game against the Tampa Bay Lightning.
Denver, also no stranger to winter storms, was the coldest it has been in 32 years on Thursday, when the temperature dropped to minus 31C in the morning at the airport.
